How to create a pointer to pointer on the Heap - c

Question

I'm struggling to create a pointer to pointer on the Heap. Here is my code:
double a = 2.289;
double** ptr = (double*) malloc(sizeof(double*));
double*** ptr_to_ptr = (double*) malloc(sizeof(double*));
I want the ptr variable to keep the address of a and ptr_to_prt the address of ptr. How can I then dereference the values. I want to do something like this:
printf("%lf", **ptr_to_ptr);
Thx in advance

Answer 1

this show how to point and check addresses of pointer on the stack
#include <stdio.h>
int main(){
double X = 2.25;
double* pX = &X;
double** ppX = &pX;
printf("Addr X: %8X\n", &X);
printf("Addr pX: %8X\n", &pX);
printf("Addr ppX: %8X\n", &ppX);
printf("Val X: %f", **ppX);
}
and this show how to point and show addresses on the heap
#include <stdio.h>
#include <stdlib.h>
int main(){
double X = 2.25;
double** pX = (double**) malloc(sizeof(double**));
double*** ppX = (double***) malloc(sizeof(double***));
*pX = &X;
*ppX = &*pX;
printf("Addr X: %8X\n", &X);
printf("Addr pX: %8X\n", &*pX);
printf("Addr ppX: %8X\n", &*ppX);
printf("Val X: %f", ***ppX);
}
you will get a space from heap and store address in them so you need pointer to pointer just for save address of 'X'
Edit 1:
for better answer to comment i put code here
below code show use & operator
'&' operator just get address of variable, so *& get value of address that variable
now look at below code bot set value of x and both correct
#include <stdio.h>
int main(){
int X;
*&X = 10;
printf("X: %d\n", X);
X = 20;
printf("X: %d\n", X);
}

Answer 2

You need an extra asterisk to have the correct type:
double a = 2.289;
double** ptr = (double**) malloc(sizeof(double*));
double*** ptr_to_ptr = (double***) malloc(sizeof(double**));
To dereference you will need as many asterisks as the amount of levels you want to go down. See the example:
int x = 5;
int *p = &x;
int **p2 = &p;
printf("%p %p\n",*p2, p); //If you use 1 asterisk you get to p
printf("%d", **p2); // If you use 2 asterisks you get what p points to, which is an int in this case